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Cable length and High Frequency Response with ISOTRON® accelerometers or In-Line Remote Charge Converter

Question

What is necessary to understand about my cable length in order to maximize my available frequency response when using ISOTRON/IEPE accelerometers?

Answer

The high frequency response of your ISOTRON/IEPE accelerometer or output run from your RCC (remote charge converter) is limited by available drive current, peak signal level and cable capacitance. These parameters are all interrelated factors that determine the frequency response of your coupling system.

ISOTRON/IEPE Accelerometer, RCC (Remote Charge Converter)

Assume that the available constant current source is adjusted to provide 8mA of constant current. Of the 8mA, assume 1mA is needed for device operation without distortion; therefore, up to 7mA are available to drive the capacitive load inherent in a cable. For instance, assume that your coaxial cable in this case is 150 ft. long. With typical cable capacitance of 32pF/ft., you will have ~4,800pF of capacitance. A scale factor of 109 is used to simply equate units. With a 5Vpk output signal from the ISOTRON/IEPE accelerometer or RCC, the highest frequency in Hz, (-3dB), that can be transmitted without current limiting is:

Note: Be cognizant of the units when calculating effects of cable length (Pico Farads), Peak Voltage (Volts) and Current (mA).

F max = max freq. Hz
Ic = current available in mA
V = Volts (pk)
C = cable capacitance in pF

Based on most available high frequency bandwidth IEPE accelerometers, this cable length would not adversely affect the intended measurement.